The audio for our light show is broadcast over an FM transmitter with attached antenna that we purchased on Amazon for ~$40. The transmitter is a Signstek 0.2Watt Stereo transmitter with adjustable frequency and output power. Amazon may not stock this exact model anymore but here is the link for reference: https://www.amazon.com/gp/product/B00IMWRK60
We connect the transmitter via a standard 3.5mm audio cable from the output on the Raspberry Pi. The transmitter has a rechargeable battery, but we just leave it plugged into power all the time.
The transmitter is mounted inside the attic near the front of our house, which gives good coverage over the street below from where people will watch our show. We did some tests with various output power levels to ensure we had a clear signal right in front of our house but did not broadcast any further than is necessary. The FCC (in the US, at least) allows for private radio transmissions up to a certain EIRP (Equivalent Isotropic Radiated Power) without a broadcasting license. I don’t know explicitly how to map that allowable output power to an equivalent allowable distance, but we operate at the minimum power necessary to accomplish our goals. Please, please, please be responsible with your radio output for your show and don’t ruin this for everyone by broadcasting too strongly and sparking interest from the FCC!
We narrowed down our list of radio frequency options by using the radio locator website (http://radio-locator.com/cgi-bin/vacant). This lets you search for channels that are vacant. You still need to verify that there is nothing actually broadcasting already on that channel. If there are multiple people in a neighborhood broadcasting for Christmas lights, it may be worth the time to coordinate stations rather than competing for signal clarity and possibly pushing your power output too high.
The only issue we had with our radio transmitter in 2016 was a dead zone in one small area in front of our house where the signal was filled with static. We assume this was due to either the metal attic vent between the transmitter and the street or possibly the bundle of cables we had run into the attic in that general direction. In 2017, we moved the transmitter to a different spot in the attic and it is doing much better. Presumably, this is because we are only broadcasting through the wooden siding and not the metal vent or a thick bundle of power cables but it is hard to be certain.